1. Field of the Invention
The present invention relates to a frost detecting device for defrosting control which is used in various industrial instruments and cold storage instruments such as refrigerators.
2. Description of the Prior Art
In general, with reference to the refrigerator, the adhesion of the frost to the surface of the cooling fin of the heat-exchanger which is incorporated into the refrigerator, deteriorates refrigerating efficiency of the cold storage instruments. If the refrigerator continues to operate on the state of adhesion of the frost remaining left as it is, not only it becomes uneconomical because of remarkable deterioration of the consumption efficiency of energy, but also it becomes a cause of a malfunction.
Accordingly, with reference to the refrigerators, a refrigerating means is operated for a definite time in such a manner that the operation lasts until the time when the time integrated reaches a fixed time. A heater is switched on in stead of the refrigerating means to defrost the refrigerator. The heater is switched off after reaching a fixed time.
In this method, a start of defrosting can be controlled by time. However, each of the states of adhesion of the frost differs mutually caused by an ambient temperature of the refrigerator, humidity, open-and-shut frequency of the door, and the state of articles such as temperature, amount of evaporation, heat capacity etc. Consequently, the state of adhesion of the frost can not be controlled by only time. Further, since this method can not detect the real state of adhesion of the frost, even if the refrigerator is in non-frost state, defrosting may be performed, contrary to the above situation, although the refrigerator is in super-frost state, defrosting may not be performed. As a result thereof, the inefficient refrigerating operation with regard to energy was performed.
Under these situations, the following frost detecting methods have been developed:
(1) As shown in FIG. 13A, optical means; a light emitting element 20, a photo detector 21 are used. When light irradiated from the light emitting element 20 reflect at the reflecting surface 22, an occurrence of adhesion of the frost is detected by detecting change in quantity of light with gaps of the angle of incidence for incident light which is directed to the photo detector 21, corresponding to the amount of adhesion of the frost or by sensing an index of reflection for incident light.
(2) This is a method in which the difference in temperature is detected. The occurrence of adhesion of the frost is detected by detecting the difference in temperature either the refrigerating means or the circumference thereof before and after adhesion of the frost.
(3) As shown in FIG. 13A, this is the method in which the change in resonant frequency of the piezoelectric vibrator is detected. The piezoelectric vibrator 25 is supported by the flexible supporting body 24 which is provided with the housing 23. The electrode 26 is mounted on the surface of the piezoelectric vibrator 25. The resonant frequency changes when the frost is adhered to the piezoelectric vibrator 25. The occurrence of adhesion of the frost is detected by detecting the change in the resonant frequency when the quantity of adhesion of the frost exceeds the fixed value, as to be the state of adhesion of the frost.
(4) The conditions which are the time integrated for operating of the compressor, open-and-shut frequency of the door, the temperature of the air outside etc. are integrated by the micro computer. The presence or absence of adhesion of the frost is judged in such a manner that the state of adhesion of the frost is presumed by the control program.
However, the usual detecting method of the adhesion of the frost has following defects.
In the optical detecting method of the above (1), it is difficult to miniaturize the detecting part. In order to maintain the detecting accuracy the periodical maintenance such as cleaning of the reflecting surface for light becomes necessary. Further, since the circuit structure thereof becomes complex, there is a defect which brings cost-up.
In the method for detecting difference in temperature of the above (2), it is difficult to obtain the accurate detecting value because the quantity of adhesion of the frost detected vary widely. Accordingly there are a lot of practical problems.
In the method using the piezoelectric vibrator of the above (3), dusts or the like may cling onto the piezoelectric vibrator. The vibration caused by the inside of the cooling instruments or the outside thereof affects the frost detecting accuracy. There is faulty operation of the detecting instruments by these effects.
In the method of the time integrated for operating of compressor and so forth of the above (4), there are differences of degree for the adhesion of the frost because of season, weather and using condition of the compressor. There may be defects that energy efficiency becomes poor if uses require, because there are differences of the degree of adhesion of the frost on account of season, weather and using the compressor.